What flying machine did Leonardo da Vinci sketch with a large spiral rotor?

Leonardo da Vinci sketched an aerial screw as a proposed flying machine with a large spiral rotor.

The design appears in Leonardo’s notebooks from the late 15th century. It consists of a helical linen surface stretched over a reed framework, intended to rotate and lift the craft through the air. Leonardo reasoned that the spiral would “screw” upward through the atmosphere, much as a screw moves through a solid material.

The aerial screw is often called an early helicopter concept, but it could not have flown as drawn. Its materials and human-powered mechanism could not produce enough lift. Modern helicopters use powered rotors and carefully shaped blades, not a continuous fabric helix.

Leonardo did not build a working aerial screw. The sketch is nevertheless important in the history of flight because it shows an attempt to apply mechanical principles to vertical lift centuries before powered aviation.